Optical structures that scatter or diffuse light generally function in one of two ways: (a) as a surface diffuser utilizing surface roughness to refract or scatter light in a number of directions; or (b) as a bulk diffuser having flat surfaces and embedded light-scattering elements.
A diffuser of the former kind is normally utilized with its rough surface exposed to air, affording the largest possible difference in index of refraction between the material of the diffuser and the surrounding medium and, consequently, the largest angular spread for incident light. However, a diffuser of this type suffers from two major drawbacks: a high degree of backscattering and the need for air contact. Backscattering causes reflection of a significant portion of the light back to the originating source when it should properly pass through the diffuser, lowering the efficiency of the optical system. The second drawback, the requirement that the rough surface must be in contact with air to operate properly, may also result in lower efficiency. If the input and output surfaces of the diffuser are both embedded inside another material, such as an adhesive for example, the light-dispersing ability of the diffuser may be reduced to an undesirable level.
In one version of the second type of diffuser, the bulk diffuser, small particles or spheres of a second refractive index are embedded within the primary material of the diffuser. In another version of the bulk diffuser, the refractive index of the material of the diffuser varies across the diffuser body, thus causing light passing through the material to be refracted or scattered at different points. Bulk diffusers also present some practical problems. If a high angular output distribution is sought, the diffuser will be generally thicker than a surface diffuser having the same optical scattering power. If however the bulk diffuser is made thin, a desirable property for most applications, the scattering ability of the diffuser may be too low.
Despite the foregoing difficulties, there are applications where an embedded diffuser may be desirable, where the first type of diffuser would not be appropriate. For example, a diffuser layer could be embedded between the output polarizer layer and an outer hardcoat layer of a liquid crystal display system to protects the diffuser from damage. Additionally, a diffuser having a thin profile, which will retain wide optical scattering power when embedded in other materials and have low optical backscatter and therefore higher optical efficiencies than conventional diffusers, would be highly desirable.